Trench semiconductor device having gate oxide layer with multiple thicknesses and processes of fabricating the same

ABSTRACT

The a trench semiconductor device such as a power MOSFET the high electric field at the corner of the trench is diminished by increasing the thickness of the gate oxide layer at the bottom of the trench. Several processes for manufacturing such devices are described. In one group of processes a directional deposition of silicon oxide is performed after the trench has been etched, yielding a thick oxide layer at the bottom of the trench. Any oxide which deposits on the walls of the trench is removed before a thin gate oxide layer is grown on the walls. The trench is then filled with polysilicon in or more stages. In a variation of the process a small amount of photoresist is deposited on the oxide at the bottom of the trench before the walls of the trench are etched. Alternatively, polysilicon can be deposited in the trench and etched back until only a portion remains at the bottom of the trench. The polysilicon is then oxidized and the trench is refilled with polysilicon. The processes can be combined, with a directional deposition of oxide being followed by a filling and oxidation of polysilicon. A process of forming a “keyhole” shaped gate electrode includes depositing polysilicon at the bottom of the trench, oxidizing the top surface of the polysilicon, etching the oxidized polysilicon, and filling the trench with polysilicon.

This application is a divisional of application Ser. No. 10/793,089,filed Mar. 4, 2004, now U.S. Pat. No. 6,900,100, which was acontinuation of application Ser. No. 09/792,667, filed Feb. 21, 2001,now abandoned, which was a continuation of application Ser. No.09/318,403, filed May 25, 1999, now U.S. Pat. No. 6,291,298. Each of theforegoing applications is incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

This invention relates to semiconductor devices having a gate electrodethat is embedded in a trench and in particular to structures and methodsof protecting such devices against damage to the gate oxide layer whenthe devices are subjected to high voltage differences while in an offcondition. The invention particularly relates to trench MOSFETs.

BACKGROUND OF THE INVENTION

There is a class of semiconductor devices in which a gate electrode isformed in a trench that extends from the surface of a semiconductorchip. One example is a trench-gated MOSFET, and other examples includeinsulated gate bipolar transistors (IGBTs), junction field-effecttransistors (JFETs) and accumulation-mode field-effect transistors(ACCUFETs). All of these devices share the common characteristic of atrench structure where the bottom of the trench for some reason can beexposed to high electric fields or where the bottom of the trench mightform a parasitic capacitor including the gate electrode and thesemiconductor material surrounding the trench.

FIGS. 1 through 10 show cross-sectional views and characteristics ofknown trench-gated devices. FIG. 1 shows a trench-gated MOSFET 100having a top metal layer 102, a gate 104 formed in a trench 106 andseparated from an epitaxial silicon layer 108 by a gate oxide layer 110.MOSFET 100 also includes an N+ source region 112 and a P-body 114. Thedrain of MOSFET 100 includes the N-epi layer 108 and an N+ substrate116. A deep P+ region 118 is created under P-body 114, as suggested inU.S. Pat. No. 5,072,266 to Bulucea et al. The PN junction between deepP+ region 118 and N-epi layer 108 forms a voltage-clamping diode 117where avalanche breakdown normally occurs. A P+ body contact region 119forms a contact between metal layer 102 and P-body 114. The gate, whichis typically formed of polysilicon, is protected from the metal layer102 by an oxide layer 120 that is above the gate 104 and that ispatterned with a feature that does not correspond to the trench itself,typically a contact mask.

As shown, gate oxide layer 110 consists of a uniform thin layer of oxidealong the three sides of the polysilicon gate 104. That is, the portionsof gate oxide layer 110 on the sidewalls of the trench and also thecurved and linear portions of the gate oxide layer 110 at the bottom ofthe trench (except for some stress-related and etch-related changes inthe oxide thickness that occur at the trench bottom) are generally of auniform thickness in the range of, for example, 150 Å to 1,200 Å.

There are many variations of this general type of MOSFET. For example,FIG. 2 shows a MOSFET 130 which is generally similar to MOSFET 100 butdoes not include a deep P+ region 118. The gate of MOSFET 130 protrudesslightly through P-body 132 because the depth of P-body 132 and thedepth of the trench 134 are determined in two unrelated processes. Thus,in vertical devices there is no guarantee of the net overlap of thepolysilicon gate into the drain region. It turns out that this variationaffects the operation of the device and may affect its reliability aswell. Also, in FIG. 2 there is no additional diode formed by the deep P+region 118 to clamp the voltage, so breakdown can occur wherever thevoltage is raised to the point that the device goes into avalanche.

MOSFET 140, shown in FIG. 3, is variation of MOSFETs 100 and 130, wherethe MOSFET cells 142 contain no deep P+ region, and a diode cell 144containing a deep P+ region is distributed at predetermined intervalsthroughout the array to act as a voltage clamp and limit the strength ofthe electric fields in the MOSFET cells. In MOSFET 140, the gate oxidelayer is of uniform thickness.

FIGS. 4A–4G illustrate various aspects of the breakdown phenomenon. FIG.4A shows the electric field strength contours at breakdown in atrench-gated device 150 having a relatively thick gate oxide layer.Device 150 is in effect a gated diode, a structural element of mosttrench-gated vertical power MOSFETs. As indicated, the strongestelectric field, where impact ionization would occur during avalanchebreakdown, is located at the junction directly beneath the P+ bodyregion. In contrast, device 160, shown in FIG. 4B, has a relatively thingate oxide layer. While some ionization still occurs underneath the P+region, the highest electric field levels are now located near thecorner of the trench. A field plate induced breakdown mechanism causesthe strength of the electric field to increase.

FIGS. 4C and 4D show the ionization contours of devices 150 and 160,respectively, when they go into avalanche breakdown. Whether there is athick gate oxide layer, as in FIG. 4C, or thin gate oxide layer, as inFIG. 4D, eventually in “deep” avalanche, i.e., when the device is forcedto conduct large currents in avalanche, breakdown starts to occur at thecorner of the trench. Even in the thick oxide case (FIG. 4C), where thepeak electric field is not at the corner of the trench (FIG. 4A), as thedrain voltage increases eventually ionization occurs at the corner ofthe trench. However, there are more contours in FIG. 4D, indicating ahigher ionization rate where the gate oxide layer is thin.

FIG. 4E shows that if one introduces a diode clamp including a deep P+region, as shown on the right-hand side, the diode will break down at alower voltage, and avalanche breakdown should not occur at the corner ofthe trench. If the resistance of the current path through the diode islow enough, then the diode will clamp the maximum voltage of the device.As a result, the voltage will never rise to the point that avalanchebreakdown occurs near the corners of the trenches.

FIG. 4F is a graph showing the breakdown voltage (BV) as a function ofgate oxide thickness (X_(OX)) for 20 V and 30 V devices. The dopingconcentration of the epitaxial (epi) layer in the 30 volt device is morelightly doped. The 30 V device would ideally have an avalanche breakdownof around 38 volts. In the 20 volt device the epi would be more heavilydoped and the device would ideally have an avalanche breakdown of around26 or 27 V. As the gate oxide is thinned from 1,000 Å to a few hundredÅ, basically the breakdown voltages are relatively constant or mayactually even increase somewhat as the shape of the field plate of thegate is actually beginning to help relax the electric field. Atthicknesses of less a few hundred Å, however, breakdown degradationbegins to occur.

Beyond the point where the breakdown voltage begins to drop (below 30 Vfor the 30 V device epi and below 20 V for the 20 V device) is the arealabeled field plate induced (fpi) breakdown. In this area, breakdownoccurs near the trench. For a reliable device one needs to add a diodeclamp having a breakdown that is lower than the breakdown in the fieldplate induced area, so that the diode breaks down first. With a diodehaving a breakdown voltage as shown in FIG. 4F, breakdown would neveroccur near the gate in the 30 V device, but that diode would have toohigh a breakdown voltage to protect a 20 V device. To protect the 20 Vdevice, the breakdown voltage of the diode clamp would have to be belowthe curve for the 20 V device.

FIG. 4G is a schematic diagram of the devices shown in FIGS. 4A–4Dshowing a gated diode in parallel with a MOSFET and a diode voltageclamp in parallel with both the MOSFET and gated diode. The arrangementis designed such that the diode clamp breaks down first. The gated diodenever “avalanches” before the diode clamp. This becomes more and moredifficult to do as the gate oxide layer becomes thinner.

FIGS. 5A and 5B show the ionization contours in a device 170 having asharp trench corner and a device 172 having a rounded trench corner.FIG. 5B indicates that rounding the trench corners does reduce themagnitude of the ionization, but ultimately if one drives the devicedeeply enough into breakdown, the breakdown still occurs at the trenchcorner, and the device is at risk.

FIGS. 6A–6C show the electric field strength contours, the equipotentiallines and the electric field lines, respectively, in a MOSFET 180. Thegate of MOSFET 180 is tied to the source and body and is grounded, andthe drain is biased at V_(D). From FIG. 6B it is evident that the drainvoltage V_(D) is divided and spaced out across the region. On the lefthand side of FIG. 6B, the equipotential lines are squeezed closertogether, and particularly around the trench corner they are squeezedeven tighter. This produces electric field lines that are at rightangles to the equipotential lines, as shown in FIG. 6C. One can see whya high electric field occurs at the trench corner and why rounding thecorner does not solve this problem. It is basically a volumetric problemin that there is an electric field that terminates on an electrodehaving a lower surface area, namely the gate, and so the electric fieldlines are crowded at the corner.

FIG. 6D shows MOSFET 180 when it is turned on by putting a positivevoltage V_(G) on the gate. A current flows down the side wall of thetrench and then it also spreads out along the bottom of the trench andinto the region below the mesa at an angle from the side of the trench.However, in the process the current flows through areas that have highelectric fields, as shown by the electric field contours of FIG. 6A.When a high current flows through an area that has a high field (andthat would be the case where the device is saturated), the currentcarriers collide with the atoms of the epi layer and knock off, bymomentum transfer, additional carriers. This forms new electron-holepairs that in turn are accelerated and create additional collisions,ionizing additional atoms.

FIG. 6E shows the ionization contours in MOSFET 180 when it is in the onstate. The ionization contours shown in FIG. 6E are different from thoseshown in FIG. 4C, for example, when device 150 is in the off state. Thedifference is that the ionization contours pull upwards all the wayaround the side of the trench, even up near the P-body. This has anumber of damaging effects on the device. One effect is that it createselectron-hole pairs in the vicinity of the gate oxide that can beaccelerated quite easily by the high electric field in that area. Theelectron-hole pairs can actually be trapped in the gate oxide, and theycan damage the gate oxide.

Furthermore, this phenomenon produces an upper limit in the amount ofvoltage that one can put on the device, because so many electron-holepairs may be produced that they begin to modulate the effective dopingconcentration of the epitaxial layer, by making the region around theside of the trench seem more heavily doped than it actually is. Thatoccurs because electrons from the newly generated electron-hole pairsare swept into the substrate by the positive drain voltage V_(D), andthe holes are swept into the P-body. The net effect is that, since theelectrons and holes can only travel at a certain velocity, the localcharge distribution adjusts itself to maintain charge neutrality.Specifically, surrounding the reverse-biased junction is a region knownas a depletion region or space charge region, where (in the absence ofimpact ionization) no free charge carriers are present. The immobilecharge residing within the depletion region, namely positive ions on theN-type side of the junction and negative ions on the P-type side of thejunction, produces a “built-in” electric field across the junction. Inthe presence of impact ionization, the holes drifting across the N-typeregion add to the positive fixed charge and thereby increase theelectric field, further enhancing the impact ionization process. Theseexcess holes make the epitaxial region, which in this example is N-typematerial, appear more heavily doped because of the increase in the“built-in” field. The net effect is an increase in the electric field,which degrades the breakdown. This effect is shown in thecurrent-voltage characteristics of FIG. 6F where the drain current I_(D)increases dramatically at a certain drain voltage. The drain voltage atwhich this happens is the same for each of the gate voltages shown. Thisproblem becomes worse as the gate oxide is thinned.

Another problem with the trench device relates to capacitance. FIG. 7Ais a schematic diagram of a MOSFET 190 having a gate driven by a currentsource 192 and having resistive load 194. A voltage source 196 connectedto the source and drain supplies a voltage V_(DD) resulting in a drainvoltage V_(D) at the drain. As shown in FIGS. 7B–7D, at a time t₁current source 192 begins to supply a constant current to the gate andthe voltage on the gate relative to the source, labeled V_(G) in FIG.7C, starts to rise. But because it does not immediately hit threshold,the drain voltage V_(D) does not start to fall because MOSFET 190 is notyet turned on. As soon as the V_(G) hits threshold, at time t₂, MOSFET192 saturates and turns on and carries current. V_(D) starts to drop,but as it starts to drop it causes a capacitive coupling between thedrain and the gate of MOSFET192 and halts the upwards progression of thegate voltage V_(G). V_(G) remains flat until MOSFET 192 gets into itslinear region. Then, MOSFET 192 begins to look like an on-resistance ina voltage divider, with a small voltage across MOSFET 192 and most ofthe voltage V_(DD) across resistor 194.

At that point the capacitive coupling effect between gate and drain issatisfied and the V_(G) continues its progress to a higher voltage. Theplateau is due to a gate-to-drain overlap capacitance similar to theMiller effect, but this is not a small signal effect. This is a largesignal effect. At that time the drain current I_(D) also continues torise, but as shown in FIG. 7D its upward progression is slowed.

FIG. 7E shows a plot of V_(G) as function of the charge on the gateQ_(G), where Q_(G) is equal to I_(G) times the time t, I_(G) being aconstant. The gate voltage V_(G) rises to a certain level, then itremains constant, and then it rises again. If there were no feedbackcapacitance between the drain and gate, the voltage would rise linearly,but instead the straight line is interrupted by the plateau.

In FIG. 7E, the point V_(G1), Q_(G1) corresponds to a certaincapacitance because C is equal to ΔQ over ΔV. Since it takes more chargeto get to the point, Q_(G2) and V_(G1), then that point reflects morecapacitance. So the capacitance in the device, as shown in FIG. 7F,starts at a low value C_(ISS), which is relatively constant, and then itjumps to a higher effective value C_(G)(eff), and then it is relativelyconstant. Because of this effect the device has a higher effectivecapacitance than is desirable during the switching transition. As aresult, there is an undue amount of energy lost in turning the deviceon.

As shown in FIG. 7G, the input capacitance actually has a number ofcomponents, including the gate-to-source capacitance C_(GS) and thegate-to-body capacitance C_(GB), neither of which exhibits theamplification effect of the gate-to-drain capacitance C_(GD). Thegate-to-drain capacitance C_(GD) is shown in FIG. 7G, around the bottomand side wall of the trench. The equivalent schematic is shown in FIG.7H. Even if C_(GD) is the same order of magnitude as C_(GS) and C_(GB),electrically it will look much larger (e.g., 5 to 10 times larger)because it is amplified during the switching process.

As indicated above, rounding the trench bottom helps to limit the damageto the gate oxide layer, although it is not a complete solution to theproblem. FIGS. 8A–8C illustrate a process for forming a trench withrounded corners. In FIG. 8A small reaction ions 202 etch the siliconthrough an opening in a mask 200 at the surface. Ions 202 areaccelerated by an electric field in a downward direction such that theyetch a trench having essentially a straight side wall. When the trenchreaches a certain depth the electric field is relaxed, as shown in FIG.8B. Alternatively, one could possibly change the chemistry. At the endof the process, as shown in FIG. 8C, the electric field is modified sothat the etching ions are traveling in all different directions. Thatbegins to not only widen the trench, but also rounds out the bottom.Hence, the process includes an anisotropic etch that is converted to anisotropic etch. The anisotropy is also influenced by the formation of apolymer as a by-product of the etching operation on the sidewall of thetrench. If the chemistry removes the polymer as soon as it forms, theetch will behave in a more isotropic way. If the polymer remains on thesidewall, only the bottom of the trench will continue to etch.

FIGS. 9A–9D show a method that includes creating a mask 210 (FIG. 9A),etching the trench 212 (FIG. 9B), forming an oxide layer 214 on thewalls of the trench (FIG. 9C), which may be removed and then re-grown toremove defects (this is called sacrificial oxidation), and then fillingthe trench with a polysilicon layer 216 (FIG. 9D).

FIGS. 10A–10F illustrate a typical process of forming a trench MOSFET.The process starts with an N-epitaxial layer 220 grown on an N+substrate 222 (FIG. 10A). Using the process of FIGS. 9A–9C, for example,a polysilicon-filled trench 224 is formed in N-epi layer 220 (FIG. 10B).The surface may or may not be planar depending on how the surface oxidesare made in the process. Then a P-body 226 is introduced, although theP-body 226 could be introduced prior to the formation of the trench 224(FIG. 10C). Both process flows are manufacturable, but forming thetrench first is preferable because the etching process can influence thedoping concentrations in the P-body. Then the surface is masked and anN+ source region 228 is implanted (FIG. 10D). An optional shallow P+region 230 is implanted to ohmic contact between the P-body and a metallayer to be deposited later. P+ region 230 can be implanted through anopening in an oxide layer 232 that is deposited across the region andthen etched to form a contact mask (FIG. 10E). The contact mask may ormay not be used to define the P+ region 232. Finally, a metal layer 234is deposited on the surface to contact the N+ source region 228 and P+region 230 (FIG. 10F).

SUMMARY OF THE INVENTION

In accordance with this invention, a trench-gated semiconductor deviceis formed, having a dielectric layer separating the gate electrode fromthe semiconductor material surrounding the trench wherein the thicknessof the dielectric layer is greater in a region at the bottom of thetrench. This structure helps to reduce the strength of the electricfield near the bottom of the trench, particularly at the corner orrounded portion where the bottom of the trench makes a transition to asidewall of the trench, and to reduce capacitance.

Several processes are used to fabricate this structure. One processincludes the following steps. A trench is etched in the semiconductormaterial. A directional deposition of a dielectric material is thenperformed such that the dielectric material is deposited preferentiallyon horizontal surfaces such as the bottom of the trench. This is done bycreating an electric field in the deposition chamber (e.g., a chemicalvapor deposition or sputtering chamber) so as to accelerate the chargedions of the dielectric towards the semiconductor material. The trench isfilled with a conductive material that will form the gate electrode.Following the directional deposition, any of the dielectric that wasdeposited on the sidewall of the trench can be removed, and a conventiondielectric layer can be grown on the sidewall of the trench. In manyprocesses, the dielectric material is silicon dioxide and the conductivematerial is polysilicon.

In one process, the conductive material is etched back to a levelroughly coplanar with the surface of the semiconductor material, and adielectric layer is deposited over the top surface of the dielectricmaterial. In one variant, the conductive material (e.g., polysilicon) isoxidized to form on oxide layer, preferably after the conductivematerial has been etched back into the trench. The conductive materialcan be oxidized to a thickness such that the oxide itself is adequate toinsulate the gate electrode, or another conductive material, such asglass, can be deposited over the oxidized conductive material.

In another variant, the conductive material that forms the gateelectrode is deposited in two stages.

In another alternative, a masking material such as photoresist isapplied after the preferential deposition of the dielectric material.The masking material is removed from all locations except the bottom ofthe trench, and the trench is subjected to an etch or dip to removedielectric material from the sidewalls of the trench. A dielectric layeris then formed on the sidewalls of the trench.

In yet another alternative, following the directional deposition of thedielectric, a material such as polysilicon that can be oxidized to forma dielectric is deposited and etched back until only a portion of thematerial remains on top of the dielectric at the bottom of the trench.The material is then oxidized to form a thicker dielectric layer at thebottom of the trench.

Another group of alternatives avoids the directional deposition of adielectric material. Instead a material such as polysilicon that can beoxidized to form a dielectric is deposited and etched back until only aportion remains at the bottom of the trench.

Processes in accordance with this invention may include a process forself aligning the trench with a contact to the top surface of the “mesa”between the trenches. A “hard” layer of a material such as siliconnitride is used as a trench mask. The hard mask remains in place until adielectric layer has been formed over the gate electrode, preferably byoxidizing polysilicon gate. The hard mask is then removed, exposing theentire top surface of the mesa and allowing a contact to be made theretowith a metal layer.

A process of this invention may include the use of a sidewall spacernear the top corners of the trench to prevent a short between the gateelectrode and the semiconductor mesa. After the trench mask has beendeposited and an opening defining the location of the trench has beenmade in the trench mask, a layer of a “hard” material such as siliconnitride, and optionally an overlying oxide, is isotropically depositedinto the opening in the trench mask. The “hard” material is deposited onthe exposed edges of the trench mask. An etch is then performed,following which the surface of the semiconductor material is exposed inthe central region of the opening but some of the deposited dielectricremains on the side edges of the trench mask, forming sidewall spacers.The trench is then etched. The dielectric sidewall spacers provideadditional insulation between the later formed gate electrode and thesemiconductor material in the mesa.

Another group of processes provide a “keyhole” shaped trench, wherein athick dielectric layer extends some distance upward on the sidewalls ofthe trench. After the trench has been etched, a relatively thick oxidelining is grown or deposited on the bottom and sidewalls of the trench.The trench is filled with polysilicon, and the polysilicon is thenetched back such that only a portion remains at the bottom of thetrench, overlying the oxide lining. The exposed oxide lining is removedfrom the sidewalls of the trench. The polysilicon is then partiallyoxidized by heating to form an oxide layer at its exposed surface, andduring the same heating process an oxide layer is formed on thesidewalls of the trench. The trench is then subjected to an oxide etch,which removes the oxide layer formed from the polysilicon as well assome of the oxide layer from the sidewalls of the trench. The trench isthen refilled with polysilicon to yield a keyhole-shaped gate electrode.

In a variant of the above process for forming a keyhole-shaped gateelectrode, after the oxide lining has been formed on the bottom andsidewalls of the trench, an amount of a masking material such asphotoresist is deposited over the oxide lining at the bottom of thetrench. An oxide etch is then performed to removed the oxide lining fromthe sidewalls of the trench, and the masking material is removed fromthe bottom of the trench. A relatively thin gate oxide layer is grown onthe sidewalls of the trench, and the trench is filled with a conductivematerial such as polysilicon which forms the gate electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art trench power MOSFEThaving a deep P+ diode which functions as a voltage clamp.

FIG. 2 is a cross-sectional view of a prior art trench power MOSFEThaving a flat body-drain junction.

FIG. 3 is a cross-sectional view of a prior art trench power MOSFEThaving a voltage clamp which is distributed among MOSFET cells whichcontain a flat body-drain junction.

FIG. 4A is a cross-sectional view showing the electric field contours ina MOSFET having a thick gate oxide layer.

FIG. 4B is a cross-sectional view showing the electric field contours ina MOSFET having a thin gate oxide layer.

FIG. 4C is a cross-sectional view showing the ionization contours in aMOSFET having a thick gate oxide layer at the onset of avalanchebreakdown.

FIG. 4D is cross-sectional view showing the ionization contours in aMOSFET having a thin gate oxide layer at the onset of avalanchebreakdown.

FIG. 4E is a cross-sectional view showing the ionization contours in adevice which contains a deep P+ region used as a voltage clamp.

FIG. 4F is a graph showing the breakdown voltage as a function of gateoxide thickness in MOSFETs fabricated in epitaxial layers havingdifferent doping concentrations.

FIG. 4G is a schematic diagram of a trench power MOSFET with ananti-parallel diode clamp.

FIG. 5A is a cross-sectional view showing ionization contours in atrench power MOSFET having a square trench corner.

FIG. 5B is a cross-sectional view showing ionization contours in atrench power MOSFET having a rounded trench corner.

FIG. 6A is a cross-sectional view showing the electric field contours ina trench power MOSFET having a flat body-drain junction.

FIG. 6B is a cross-sectional view showing the equipotential lines in atrench power MOSFET having a flat body-drain junction.

FIG. 6C is a cross-sectional view showing the electric field lines in atrench power MOSFET having a flat body-drain junction.

FIG. 6D is a cross-sectional view showing the current flow lines in atrench power MOSFET having a flat body-drain junction.

FIG. 6E is a cross-sectional view showing the ionization contours in atrench power MOSFET when it is turned on.

FIG. 6F is a graph showing a family of I–V curves for a power MOSFET atdifferent gate voltages, showing how the sustaining voltage is reducedby impact ionization.

FIG. 7A is a schematic diagram of a gate charging circuit for a powerMOSFET.

FIG. 7B is a graph illustrating the step function application of a gatedrive current to a power MOSFET.

FIG. 7C is a graph illustrating how the gate voltage and drain voltagevary under the conditions of FIG. 7B.

FIG. 7D is a graph showing how the drain current varies under theconditions of FIG. 7B.

FIG. 7E is a graph showing how the gate voltage varies as a function ofcharge.

FIG. 7F is a graph showing how the effective input capacitance varies asa power MOSFET is turned on.

FIG. 7G is a cross-sectional view showing the components of the gatecapacitance in a trench power MOSFET.

FIG. 7H is an equivalent circuit diagram of a trench MOSFET showing theinter-electrode capacitance.

FIGS. 8A–8C are cross-sectional views showing how a gate trench havingrounded corners is formed.

FIGS. 9A–9D are cross-sectional views showing a process of etching agate trench and filling the trench with polysilicon.

FIGS. 10A–10F are cross-sectional views of a process of fabricating aconventional trench power MOSFET.

FIG. 11A is a cross-sectional view of a trench power MOSFET having athick oxide layer at the bottom of the trench.

FIG. 11B is a cross-sectional view showing the MOSFET of FIG. 11A havinga thick oxide layer patterned on the top surface of the semiconductor.

FIG. 11C is a cross-sectional view of the power MOSFET of FIG. 11A witha thick overlying oxide layer that is aligned to the walls of thetrench.

FIG. 12 is a schematic flow diagram showing a number of processsequences in accordance with this invention.

FIGS. 13A–13N illustrate a process sequence for fabricating a trenchpower MOSFET having a thick oxide layer at the bottom of the trench,using a directional deposition of an oxide layer and etching thepolysilicon to a level even with the top of the semiconductor material.

FIGS. 14A–14F illustrate an alternative process sequence in which thepolysilicon is etched to a level below the surface of the semiconductormaterial and then oxidized.

FIGS. 15A–15F illustrate an alternative process sequence in which thepolysilicon is deposited in two stages.

FIGS. 16A–16E illustrate an alternative process in which a small amountof photoresist is used to mask the thick oxide at the bottom of thetrench.

FIGS. 17A–17F illustrate a process in which the polysilicon is etched toa level near the bottom of the trench and then oxidized.

FIGS. 18A–18F illustrate an alternative process in which the polysiliconis oxidized.

FIGS. 19A–19L illustrate a process of fabricating a trench power MOSFEThaving an oxide layer over the gate electrode which is self-aligned withthe walls of the trench.

FIGS. 20A–20F illustrate a process sequence for fabricating a trenchgate in an active array portion of a power MOSFET as well as a gate bus.

FIGS. 21A–21E illustrate a problem that can occur from undercutting thethin oxide layer below the nitride.

FIGS. 22A–22C illustrate further examples of this problem.

FIGS. 23A–23G illustrate other problems that can arise in thefabrication of power MOSFETs in accordance with this invention.

FIGS. 24A–24F illustrate problems that can occur from undercutting ahard mask during the removal of the top oxide in a self-aligned device.

FIGS. 25A–25H illustrate a process of fabricating a trench power MOSFETwith a thick bottom oxide layer and a nitride side spacer.

FIGS. 26A and 26B illustrate a problem that can occur during theformation of the gate oxide layer in a thick bottom oxide device.

FIGS. 27A–27D illustrate a method of avoiding the problem illustrated inFIGS. 26A and 26B.

FIGS. 28–33 illustrate different types of trench power MOSFETs that canbe fabricated in accordance with this invention.

FIG. 34 illustrates a flow diagram of a process sequence of fabricatinga trench power MOSFET using a conventional contact mask andincorporating a thick bottom oxide layer.

FIGS. 35A–35L illustrate cross-sectional views showing the process ofFIG. 34.

FIGS. 36–39 are cross-sectional views showing trench power MOSFETshaving “keyhole” shaped gate electrodes.

FIGS. 40A–40L illustrate a process sequence for fabricating a MOSFEThaving a keyhole-shaped gate electrode.

FIGS. 41A–41F illustrate an alternative process sequence of fabricatinga MOSFET having a keyhole-shaped gate electrode.

FIGS. 42A–42C illustrate the strength of the electric field in aconventional power MOSFET, a power MOSFET having a thick bottom gateoxide, and a power MOSFET having a keyhole-shaped gate electrode,respectively.

DESCRIPTION OF THE INVENTION

The problems associated with interactions between the gate and the drainof a MOSFET can be solved in part by reducing the coupling capacitancebetween them. In accordance with this invention, this is done bythickening the gate oxide layer at the bottom of the trench. FIGS. 11–27show various structures and sequences for forming a thick gate oxide onthe bottom of the trench.

FIG. 11A shows an epitaxial (“epi”) layer 242 grown on a substrate 240.A trench 250 is formed in epi layer 242. A gate oxide layer 244 linesthe walls of trench 250, and a thick portion 246 of gate oxide layer 244is located at the bottom of trench 250. Trench 250 is filled withpolysilicon 248. Note that there is no oxide layer on top of polysilicon248. The arrangement of FIG. 11A could be an intermediate structure; anoxide layer could be formed on top of polysilicon 248 at a later stageof the process. Polysilicon 248 is typically doped to a heavy dopingconcentration. It may be formed with a top surface substantially planar,i.e., flat, with the silicon epi surface by a number of means. Onemethod to make the surface flat is to deposit the polysilicon layer to agreater thickness and then etch it back. Another means to produce a flatsurface is to deposit the polysilicon to a thickness greater than theamount needed to fill the trench and then chemical mechanically polishthe surface flat. A flat surface is desirable to reduce the height ofsteps which may form later in the fabrication process.

FIG. 11B shows a structure with an oxide layer 252 on top of polysiliconlayer 248. Since the lateral edges of oxide layer 252 do not correspondto the walls of trench 250, oxide layer 252 is most likely formed with amask and etching step. Oxide layer 252 could be either deposited (e.g.,by chemical vapor deposition) or it could be thermally grown or somecombination of these steps. FIG. 11C shows a top oxide layer 254 that isgrown in accordance with the teachings of application Ser. No.09/296,959, which is incorporated herein by reference in its entirety.The sides of oxide layer 254 are generally aligned with the walls oftrench 250 and oxide layer 254 extends down into trench 250. Polysiliconlayer 248 is thus embedded in trench 250. The embodiments of FIGS. 11Band 11C both have a thick gate oxide region 246 at the bottom of thetrench.

FIG. 12 is a schematic diagram of several process flows that can be usedto fabricate gate trenches in accordance with this invention. Thedetails of these process flows are shown in FIGS. 13–20. FIG. 12illustrates in block diagram form that the trench may be formed using aphotoresist mask or a hard mask sequence, followed by a directed oxidedeposition planarized by either a selective etch, a dipback, or aselective oxidization. A selective oxidization can be used without adirected deposition. Finally, the trench is filled with polysiliconusing a one-step or two-step process.

More specifically, starting at the left side of FIG. 12, there are twooptions for forming the trench. In one option, shown in FIGS. 13–18, thetrench is formed using a mask that is later removed, so that the mask isnot available as a reference for other processing steps. The otheroption is to use a “hard” mask to form the trench, as described in theabove-referenced application Ser. No. 09/296,959, which is then employedas a reference later in the process. This option is generally describedin FIGS. 19 and 20. After the trench is formed, normally a sacrificialoxide layer is grown on the walls of the trench and then removed. Anoxide lining may then be formed on the walls of the trench. This stageyields a trench having a uniform oxide layer on its walls, with orwithout a hard mask on the top surface of the silicon.

One may then proceed to what is called the directed dielectricdeposition, which involves depositing more oxide on the bottom of thetrench than on the sidewalls of the trench. There are then threechoices. As shown in FIG. 16, a selective etchback can be performed,allowing thick oxide to remain at the bottom of the trench and removingthe oxide from the sidewalls of the trench. As shown in FIGS. 13–15 onecan perform a “dipback” to remove the oxide layer from the sidewalls ofthe trench. Finally, one can perform a selective oxidation, as shown inFIGS. 17A and 18, wherein a polysilicon layer is formed at the bottom ofthe trench and then oxidized to form additional oxide at the bottom ofthe trench. The selective oxidation of a polysilicon layer can beperformed instead of or in addition to the directed dielectricdeposition.

At this stage of the process a trench has been fabricated with a thickoxide layer on the bottom. There may or may not be a “hard” mask on thetop surface of the semiconductor. Next, a thin oxide layer is grown onthe walls of the trench and the trench is filled with polysilicon. Thepolysilicon may be deposited as a single layer or it can be deposited astwo layers with an etchback between the depositions. Depositing thepolysilicon in a two-stage process maybe beneficial to the introductionof dopants into the “mesa” between the trenches, and to make a morelightly doped polysilicon layer available on the surface of the wafer toproduce diodes, resistors, and other polysilicon devices.

Finally a glass layer is deposited and contact openings are formed inthe glass layer.

FIGS. 13A–13N illustrate a process using the oxide “dipback” method. Theprocess starts with an epi layer 262 formed on a substrate 260. A masklayer 264 is formed on the top surface of epi layer 262, with an openingwhere the trench is to be formed. Mask layer 264 may be photoresist orsome other material and may be formed on top of an oxide layer 266. Atrench 268 is then formed using conventional processes, as shown in FIG.13A.

In FIG. 13B a sacrificial oxide layer 270 has been formed on the surfaceof the trench. Sacrificial oxide layer 270 is then removed, as shown inFIG. 13C. Sacrificial oxide layer 270 could be from 100 Å to 1000 Åthick; typically, it would be in the range of 300 Å thick. It can beformed by heating the structure at 800° C. to 1100° C. for 10 minutes tofive hours in an oxidizing ambient. The ambient could be either oxygenor it could be oxygen and hydrogen. If the ambient is a combination ofoxygen and hydrogen, it is considered a “wet” oxidation because thereaction would produce water vapor and this would affect the consistencyand growth rate of the oxide.

Optionally, an oxide lining 272 is then formed on the walls of trench268. Lining 272 could have a thickness in the range of 100 Å TO 600 Å.Lining 272 prevents the deposited oxides from contacting the silicondirectly, which would have the potential for charged states, especiallyat the interface between the silicon and the deposited oxide. Adding aclean oxide layer on the walls of the trench provides a reduced chargestate.

As shown in FIG. 13E, an electric field is applied above the surface ofepi layer 262, and dielectric ions are formed and directed downward intotrench 268 by means of the electric field. Preferably, a plasma-enhancedchemical vapor deposition chamber is used for this process. The electricfield accelerates the dielectric ions downward so that theypreferentially deposit on horizontal surfaces, including the bottom oftrench 268. The chemical vapor deposition of oxide involves a gaseouschemical reaction of oxygen and silane, dichlorosilane, or silicontetrachloride. The source of oxygen is typically nitreous oxide, andsilane is typically the silicon source. Plasma-enhanced chemical vapordeposition machines are available from such companies as NovellusSystems and Applied Materials.

Another method to achieve a directional deposition is to sputter a oxidefilm from an oxide-coated target onto the wafer. Since sputtering is amomentum transfer process, the deposition occurs in a straight line.

The result of this process is shown in FIG. 13F, where an oxide layer270 has been formed inside and outside the trench 268. Note that oxidelayer 270 is thicker at the bottom of trench 268 than on the sidewallsof trench 268. It is also thicker on the flat surfaces of epi layer 262.Processes other than chemical vapor deposition, such as sputtering,could also be used to produce oxide layer 270.

Layer 270 could be formed of materials other than oxide, such asphosphorus-doped glass or boron phosphorus silicon glass. It could alsoconsist of other materials having a low dielectric constant K, such aspolymers or polyimide. Air bubbles could be incorporated in layer 270 toreduce its dielectric constant.

In FIG. 13G, oxide layer 270 has been etched back or dipped back toremove the portions on the sidewalls of trench 268. A bottom portion 274of oxide layer 270 remains at the bottom of trench 268. As shown in FIG.13H, the structure is then heated to form a thin oxide layer 276 on thesidewalls of trench 268. A polysilicon layer 278 is then deposited tofill trench 268 and overflow the top surface of the structure. This isshown in FIG. 13I.

As shown in FIG. 13J, polysilicon layer 278 is then etched back until itis roughly coplanar with the top surface of epi layer 262. Next, theportions of oxide layer 270 on the surface of epi layer 262 are removed,taking care not to etch too much of the oxide layer 276 on the sidewallsof the trench. The result of this step is shown in FIG. 13K. Avoidingthe removal of oxide layer 276 is best performed by having polysiliconlayer 278 protrude slightly above the oxide layer 276. In FIG. 13L, theentire top surface of the structure, including the top surface ofpolysilicon layer 278, has been oxidized to form an oxide layer 280.

As shown in FIG. 13M, a glass layer 282 is laid down over the surface ofoxide layer 280, and glass layer 282 and oxide layer 280 are thenpatterned and etched to form contact openings to the epi layer 262,yielding the structure shown in FIG. 13N.

FIGS. 14A–14F show an alternate process flow beginning with thestructure shown in FIG. 13I. FIG. 14A corresponds to FIG. 13I.Polysilicon layer 278 is etched back, as shown in FIG. 14B, and then thetop surface of the remaining portion of polysilicon layer 278 isoxidized to form an oxide layer 290, as shown in FIG. 14C. A glass layer292 is then deposited over the entire surface of the structure, as shownin FIG. 14D. A mask layer 294 is formed on the top surface of glasslayer 292, and layers 270 and 292 are etched to form contact openings,as shown in FIG. 14F. Mask layer 294 is then removed.

FIGS. 15A–15F illustrate yet another alternative process, beginningagain with the structure shown in FIG. 13I. FIG. 15A corresponds to FIG.13I. Polysilicon layer 278 is etched back to a level inside the trench,as shown in FIG. 15B. Next, a second polysilicon layer 300 is depositedover the entire structure, as shown in FIG. 15C. Polysilicon layer 300is then etched back, but care is exercised to ensure that the portion ofoxide layer 276 at the upper corner of the trench is not exposed. Theresulting structure is shown in FIG. 15D. Next, oxide layer 270 isremoved, as shown in FIG. 15E, and an oxide layer 302 is formed over theentire surface of the structure. A glass layer 304 is then depositedover oxide layer 302, yielding the structure illustrated in FIG. 15F.

FIGS. 16A–16E illustrate an alternative process, beginning with thestructure in FIG. 13F. FIG. 16A corresponds to FIG. 13F. A photoresistlayer is then formed over the structure and is developed and rinsed in away that is sufficient to clean the photoresist layer off the top of thestructure but leave it at the bottom of trench 268. This takes advantageof the fact that it is difficult to get the photoresist out of thebottom of the trench 268. The resulting structure with a remainingportion of photoresist layer 310 in the bottom of trench 268 is shown inFIG. 16B. An oxide etch is then performed removing the portion of oxidelayer 270 from the sidewalls of trench 268. A thorough rinse is thenperformed to remove photoresist 310, producing the structure illustratedin FIG. 16C. The structure is then oxidized to form a thin oxide layer312 on the sidewalls of the trench and the trench is filled with apolysilicon layer 314, as shown in FIGS. 16D and 16E. A two-steppolysilicon deposition could be performed as shown in FIGS. 15A–15C.

FIGS. 17A–17F show yet another alternative process sequence, beginningwith the structure shown in FIG. 13F. FIG. 17A corresponds to FIG. 13F.As shown in FIG. 17B, a sacrificial polysilicon layer 320 is deposited.Polysilicon layer 320 is etched back until only a small portion 322remains at the bottom of trench 268. The portion 322 of polysiliconlayer 320 is then oxidized. A low temperature oxidation process is used(e.g., 700 to 950° C.), since at a low temperature polysilicon oxidizesmore rapidly than single crystal silicon. Thus oxide forms in portion322 at a faster rate than on the sidewalls of trench 268. The resultingstructure is shown in FIG. 17D, with an oxide layer 324 at the bottom oftrench 268. The portion of oxide layer 270 is removed from the sidewallsof trench 268, as shown in FIG. 17E, and a thin gate oxide layer 326 isformed on the sidewalls of trench 268, as shown in FIG. 17F.

FIGS. 18A–18F show yet another alternative process sequence, beginningwith the structure shown in FIG. 13B. FIG. 18A corresponds to FIG. 13D,where oxide lining 272 has just been formed. Instead of using adirectional dielectric deposition, as shown in FIG. 13E, a sacrificialpolysilicon layer 330 is deposited, as shown in FIG. 18B. Polysiliconlayer 330 is etched back until only a small portion 332 remains at thebottom of trench 268, as shown in FIG. 18C. The structure is thensubjected to a low-temperature oxidation, as described above, convertingpolysilicon portion 332 into an oxide layer 334, as shown in FIG. 18D.Oxide lining 272 is then stripped from the sidewalls and top surfaces ofthe structure, as shown in FIG. 18E, and a gate oxide layer 336 is grownon the sidewalls of trench 268. The resulting structure is then shown inFIG. 18F.

FIGS. 19A–19I illustrate a process which contains elements of the superself-aligned process described in the above-referenced application Ser.No. 09/296,959. The structure is formed in an epi layer 342 which isgrown on a substrate 340. A thin oxide layer 346 is formed on thesurface of epi layer 342, and this is covered by a layer 344 of a hardmasking material such as silicon nitride. An opening is etched innitride layer 344 and oxide layer 346, as shown in FIG. 19A.

As shown in FIG. 19B, a trench 348 is etched in epi layer 342 using aconventional process. A sacrificial oxide layer is (not shown) formed onthe walls of trench 348 and then removed. As shown in FIG. 19C, an oxidelining 350 is then formed on the walls of trench 348. As shown in FIG.19D, a directional deposition of the kind described above in connectionwith FIG. 13E is performed, forming an oxide layer 352. Oxide layer 352includes a thick portion 254 at the bottom of trench 348. As shown inFIGS. 19E and 19F, the portions of oxide layer 352 and oxide lining 350are removed from the sidewalls of trench 348. This is done by dippingthe structure in, for example, 170 HF acid. A gate oxide layer 356 isthen formed and the trench is filled with a polysilicon layer 358. Thesesteps are shown in FIGS. 19G and 19H.

As shown in FIG. 19I, polysilicon layer 358 is then etched back to alevel above the surface of the thin oxide layer 346. In FIG. 19J, thethick oxide layer 352 has been removed from above the nitride layer 344,with the polysilicon layer 358 protecting the thin oxide layer 356 atthe edges of trench 348. The structure is then annealed such that aportion of polysilicon layer 358 is oxidized to form a thick oxide layer360 in the upper region of the trench, as shown in FIG. 19K. Finally, asshown in FIG. 19L, nitride layer 344 is removed.

FIGS. 20A–20F show a two-stage polysilicon process with two trenches,one of which is in the active array and the other of which is part of agate bus. The process starts at the point illustrated in FIG. 19H, witha polysilicon layer 388 filling trenches 374A and 374B. A thick oxidelayer 384 has been formed at the bottom of trenches 374A and 374B. Asilicon nitride layer 374 overlies the surface of epi layer 372. Nitridelayer 374 is covered by an oxide layer 382.

Polysilicon layer 388 is etched back as shown in FIG. 20B, and oxidelayer 382 is removed. A second polysilicon layer 390 is deposited overpolysilicon layer 388, and a “hard” layer 392, formed of nitride orpolyimide, for example, is deposited on top of the second polysiliconlayer 390. The resulting structure is illustrated in FIG. 20C.

As shown in FIG. 20D, polysilicon layer 390 and the hard layer 392 areetched from the region of the active array (trench 374A), leaving theselayers in the region of the gate bus (trench 374B). The structure isthen heated to oxidize polysilicon layer 388 in trench 374A producing athick oxide layer 394 in the upper region of that trench. At the sametime, an oxide layer 396 forms on the exposed edge of second polysiliconlayer 390. This structure is shown in FIG. 20E.

Finally the exposed portions of the hard layers 374 and 392 are removed,yielding the arrangement shown in FIG. 20F.

FIGS. 21A–21E and 22A–22C illustrate two problems that need to beavoided. FIG. 21A shows a sacrificial oxide layer 400 along the walls ofthe trench and a thin oxide layer 404 and a nitride layer 402 on the topsurface of the epi layer. As shown in FIG. 21B, in the process ofremoving the sacrificial oxide layer 400, a portion of the thin oxidelayer 404 has been removed underneath nitride layer 402. The solution tothis problem is to minimize the oxide overetch time or to use an oxidelayer 404 that is as thin as possible, even as thin as 15 to 90 Å.

When the gate oxide layer 406 is formed, following the formation of athick oxide layer 408 at the bottom of the trench, the gate oxide layer406 may not adequately cover the upper corner of the trench, as shown inFIG. 21C. FIGS. 21D and 21E show the arrangement after a polysiliconlayer 412 has been deposited and, using a “hard” layer 414, formed ofnitride or polyimide, for example, as a mask, etched back from an activearray area of the device, showing the thin oxide layer that separatespolysilicon layer 412 from the epi layer at the upper corner of thetrenches.

FIGS. 22A–22C illustrate another potential problem area. FIG. 22A showsa device at the same stage that is illustrated in FIG. 19D, with thickoxide layer 352 having been directionally deposited, forming a thickportion 354 at the bottom of the trench. In the process of removing theoxide from the sidewalls of the trench, as shown in FIG. 22B, a portionof the thin oxide layer 346 is removed from underneath nitride layer344. Then, when the gate oxide layer 356 is grown, the portion of theoxide layer at the upper corner of the trench is unduly thin, and thiscan lead to defects in the oxide and shorting between the gate and theepi layer. This problem is illustrated in FIG. 22C. Again, the solutionis to minimize any oxide overetch or alternatively to use a plasma etchwhose chemistry etches isotropically.

FIG. 23A shows a problem that can result when the polysilicon fills acavity that is formed under the nitride layer, as shown in FIG. 21E. Aportion 420A of polysilicon layer 420 extends outside the trench andwill form a short to a metal layer deposited later to contact the epilayer. During oxidation, the oxide 422 does not consume the siliconfilling under the nitride overhang. Removal of the nitride exposes thegate to a source metal short. FIG. 23B shows a variation in which theportion 420B is separated by oxide from the main polysilicon layer 420.FIG. 23C illustrates a case in which polysilicon layer 420 has formedupward projecting spikes 420C, creating the likelihood of a shortbetween the gate polysilicon layer 420 and a later deposited metallayer. Again, the polysilicon filling under the nitride remains afteroxidation leaving a possible gate-to-source short.

FIG. 23D shows the gate I–V characteristic of a shorted device. The lowresistance is referred to as a “hard” short. FIG. 23E shows thecharacteristics of a “soft” or diode-like short. Unlike the hard shortthat occurs by a direct contact of metal to the top of the polysilicongate, the diode-like short can occur within a gate bus region as shownin FIG. 23F. In this type of failure an N+ region or plume is doped intothe P body wherever the polysilicon touches the silicon mesa, producinga parasitic diode and MOSFET shown schematically in FIG. 23G.

FIGS. 24A–24F illustrate the processing mechanism that causes the diodeshort as an overetch first polysilicon layer or a misshapen, distortedtrench. In FIG. 24A, the active cell and gate bus region are filled witha first layer of N+ doped polysilicon and are then etched back toproduce the structure shown in FIG. 24B. If the etchback of thepolysilicon is nonuniform, one side of the trench oxide may be exposed,as shown in FIG. 24C, which then is attacked and etched during dip whichremoves the top oxide. In FIG. 24D, the second polysilicon layer isdeposited and patterned by a mask, leaving the active cell on the leftand the gate bus on the right. After top oxidization, shown in FIG. 24E,the active cell on the left oxidizes and heals itself, but in the gatebus region the polysilicon touching the silicon dopes an N+ plumeleading to the diode-like gate short of FIG. 24F. Uniform etchback ofthe polysilicon and uniformly shaped trenches avoid this problem.

FIGS. 25A–25H describe a process for avoiding these problems by the useof a nitride sidewall spacer. The process starts with an epi layer 502that is grown on a substrate 500. A thin oxide layer 504 is grown on thetop surface of epi layer 502 and a nitride layer 506 (or some other“hard” layer) and a second oxide layer 508 are formed in succession overoxide layer 504. Thus layers 504, 506 and 508 form anoxide-nitride-oxide (ONO) sandwich, well known in the field. Theresulting structure is illustrated in FIG. 25A.

As shown in FIG. 25B, an opening is etched in the ONO sandwich. Anitride layer 510 is then deposited over the top of the structure,yielding the arrangement shown in FIG. 25C. Nitride layer 510 is etchedanisotropically. Since the vertical thickness of nitride layer 510 ismuch greater near the edges of the ONO sandwich, the anisotropic etchleaves sidewall spacers 512 at the exposed edge of oxide layer 504 andnitride layer 506. This structure, following the removal of oxide layer508 is shown in FIG. 25D.

As shown in FIG. 25E, a trench 514 is then etched, and the typicalsacrificial gate oxide layer (not shown) is formed and removed. FIG. 25Fshows the structure after the directional deposition of an oxide layer516, which leaves a thick oxide portion 518 at the bottom of the trench514. This is done after the formation of a gate oxide layer 520. Thetrench is then filled with a polysilicon layer 522, which is etchedback, taking care not to attack the underlying oxide layer 520. The topregion where the polysilicon and silicon nearly touch will be oxidizedfurther later in the process. Also, some oxide will grow under thenitride sidewall cap, like a “bird's beak”. This structure is shown inFIG. 25G. Oxide layer 516 is then removed, producing the embodimentshown in FIG. 25H.

As shown in FIGS. 26A and 26B, growing the gate oxide on the sidewallsof the trench can lead to a “kink” in the sidewall of the trench, shownas kink 530 in FIG. 26B. The problem is that, as shown in FIG. 26A, theoxide grows uniformly on the exposed sidewall 532 of the trench.However, where the thick oxide 534 begins at the bottom of the trench,owing to the geometry of the structure, the oxidation does not proceedin a linear fashion. This creates a reduced thickness of the oxide layerat kink 530.

A solution to this problem is illustrated in FIGS. 27A–27D. FIG. 27Ashows the structure after the thermal growth of an oxide lining 540 andthe directional deposition of an oxide layer 542, as described above.Lining 540 and layer 542 are removed from the sidewall of the trench asshown in FIG. 27B. The structure is then dipped in 170 HF acid. Sinceoxide deposited by deposition etches faster than thermally grown oxide,the structure appears as in FIG. 27C following the dip, with the topsurface of lining 540 being slightly above the top surface of the oxidelayer 542. When the gate oxide layer is thermally grown on the sidewallsof the trench, the resulting oxide is of a relatively uniform thickness.There is no “kink” in the wall of the trench. FIG. 27D shows thearrangement after a gate oxide layer 544 has been grown on the sidewallof the trench. The dotted line indicates the original position of thesilicon prior to oxidation.

FIGS. 28–33 illustrate various devices that can be fabricated using theprinciples of this invention.

FIG. 28 shows a power MOSFET having a flat-bottomed P-body region and anN buried layer at the interface between the epi layer and substrate.FIG. 28 illustrates a device combining the thick trench bottom oxidewith a contact that extends entirely across the mesa between trenchesalthough a contact mask and nonplanar top oxide layer could be utilized.FIG. 29 shows a MOSFET that is similar to the one shown in FIG. 28except that each MOSFET cell contains a deep P+ region, in accordancewith the teachings of U.S. Pat. No. 5,072,266 to Bulucea et al. Theembodiment of FIG. 30 has a flat-bottomed P-body region in the MOSFETcells as well as a diode cell containing a deep P+ region which is usedto voltage-clamp the MOSFET cells. This type of arrangement is taught inapplication Ser. No. 08/846,688, incorporated herein by reference.

In the device shown in FIG. 31 there is no contact between the P-bodyregion and the overlying metal layer in the individual MOSFET cells.Instead, the body is contacted in the third dimension, as taught in U.S.Pat. No. 5,877,538 to Williams et al., which is incorporated herein byreference. Note that one of the MOSFET cells contains a deep P+ regionto limit the strength of the electric field at the bottoms of thetrenches. Again, the planarized top oxide layer using a self-alignedcontact is preferred but not required.

In the embodiment of FIG. 32 the trenches extend into the N-buried layerso that only the thick oxide region overlaps the heavily doped buriedlayer.

The embodiment of FIG. 33 is an accumulation mode MOSFET (ACCUFET), suchas the one taught in U.S. Pat. No. 5,856,692 to Williams et al., whichis incorporated herein by reference.

FIG. 34 is a conceptual drawing showing a process flow for a trenchMOSFET using a conventional contact mask and incorporating a thicktrench bottom oxide. The steps of the process generally include theformation of the drain and deep P+ regions, the etching of the trenchand formation of the gate, the implantation of the body and sourceregions, and the opening of contacts and deposition of a metal layer. InFIG. 34 the boxes with the corners clipped represent optional steps.Thus, the introduction of a deeper body region by implant or by implantand diffusion is consistent with this process.

This process is illustrated in FIGS. 35A–35L. A trench 552 is formed inan N epi layer 550, using an oxide layer 554 as a mask. An oxide lining556 is formed on the walls of the trench 552 (FIG. 35B), and adirectional oxide deposition is carried out as described above, formingan oxide layer 558 having a thick portion 560 at the bottom of thetrench (FIG. 35C). The sidewalls of trench 552 are then etched (FIG.35D), and a gate oxide layer is thermally grown on the walls of trench552 (FIG. 35E).

A polysilicon layer 564 is then deposited to fill the trench 552 (FIG.35F). Polysilicon layer 564 is etched back into the trench (FIG. 35G).An oxide layer 566 is deposited over the top surface of the structureand extends down into the trench to the top surface of polysilicon layer564 (FIG. 35H). Oxide layer 566 is then etched back (FIG. 35I), and aP-type dopant such as boron is implanted to form P body region 568. Thetop surface is then masked (not shown), and an N-type dopant such asarsenic or phosphorus is implanted to form N+ source regions 570.Another oxide layer 572 is deposited on the top surface and patterned,yielding the structure shown in FIG. 35L. The contact can then be filledby the top metal or alternatively filled with a planarizing metal suchas tungsten first, or with a barrier metal such as Ti/TiN.

FIGS. 36–39 illustrate several embodiments in which the polysilicon gateis “keyhole” shaped in cross-section. The thicker gate oxide extends notonly along the bottom of the trench but also along the sidewalls of thetrench towards the junction between the P body region and the N epilayer. The thickened gate oxide along the sidewalls of the trench helpsto soften the electric field at that junction.

FIG. 36 shows a MOSFET having flat-bottomed P body regions and a diodecell incorporated at periodic intervals among the MOSFET cells. In thepreferred version of this MOSFET a keyhole-shaped gate is employed. FIG.37 shows an embodiment where the P body does not extend to the surfacebut instead is contacted in the third dimension. A shallow P+ region isshown within the mesa at a depth greater than the N+ source regions.FIG. 38 shows an embodiment wherein the trenches extend into an N buriedlayer formed at the interface between the epi layer and the substrate.FIG. 39 shows an embodiment where the P body is contacted in the thirddimension, and the trenches extend into an N buried layer.

A process sequence for forming a device having a keyhole shaped trenchis illustrated in FIGS. 40A–40L. The process starts with an epi layer602 grown on a substrate 600. An oxide layer 604 is formed at the topsurface of epi layer 602, as shown in FIG. 40A. Oxide layer 604 ispatterned and a trench 606 is etched, as shown in FIG. 40B. Asacrificial oxide layer (not shown) is formed on the walls of the trenchand removed. An oxide lining 608 is then grown on the walls of trench606 (as shown in FIG. 40C).

As shown in FIGS. 40D and 40E, a polysilicon layer 610 is deposited tofill the trench 606 and then etched back such that a portion 612 remainsat the bottom of the trench. The oxide lining 608 is then etched fromthe walls of the trench 606, as shown in FIG. 40F. An anisotropicsilicon etch is then performed to depress the top surface of polysiliconportion 612 below the top surface of oxide lining 608, as shown in FIG.40G. A thermal oxidation process is then applied, forming an oxide layer616 on the walls of the trench 606 and an oxide layer 618 at the topsurface of polysilicon portion 612. The result is shown in FIG. 40H.Oxide layer 618 is then etched, a portion of oxide layer 616 beingremoved in the process, producing the structure shown in FIG. 40I.

A second polysilicon layer 619 is then deposited over the entirestructure, as shown in FIG. 40J. Polysilicon layer 619 is etched back,as shown in FIG. 40K. The top surface of polysilicon layer 619 is thenoxidized, as shown in FIG. 40L.

A variation of this process is illustrated in FIGS. 41A–41F. After theoxide lining 608 has been formed on the walls of the trench, as shown inFIG. 40C, a photoresist layer is applied, developed, and washed away,leaving only a portion 630 at the bottom of the trench 606. This isshown in FIG. 41A. Oxide lining 608 is then etched from the walls of thetrench 606, as shown in FIG. 41B and the portion 630 of the photoresistlayer is removed from the bottom of the trench. This yields thestructure shown in FIG. 41C.

A gate oxide layer 632 is thermally grown on the walls of trench 606,and trench 606 is filled with a polysilicon layer 634, as shown in FIGS.41D and 41E. Polysilicon layer 634 is etched back to the level of thetop surface of the epi layer 602. Polysilicon layer 634 is then oxidizedthermally to produce the device shown in FIG. 41F.

FIGS. 42A–42C show a comparison of the strength of the electric fieldalong the sidewall of the trench in a prior art trench device with thestrength of the electric field in embodiments of this invention. FIG.41A shows that in the prior art device the electric field has two sharppeaks which occur, respectively, at the body-drain junction and thebottom of the gate electrode. FIG. 42B shows a device having a thickoxide layer on the bottom of the trench. As indicated, the electricfield still has a sharp peak at the body-drain junction but the peak atthe bottom of the gate electrode is somewhat lower than in the prior artdevice. Finally, FIG. 42C shows a device having a keyhole shaped gateelectrode. In this case, the electric field still reaches a peak at thebody-drain junction, but the sharp peak at the bottom of the gateelectrode is eliminated.

While numerous embodiments in accordance with this invention have beendescribed, it will be understood that these embodiments are illustrativeonly and not limiting of the broad scope or the broad principles of thisinvention.

1. A process of fabricating a trench semiconductor device comprising:providing a semiconductor material; forming a mask layer on a surface ofsaid semiconductor material; forming a opening in said mask layer;forming a trench in said semiconductor material by etching saidsemiconductor material through said opening; forming an oxide lining ona bottom and sidewalls of said trench; depositing a first polysiliconlayer in said trench; etching said first polysilicon layer such that aremaining portion of said first polysilicon layer remains over saidoxide lining at the bottom of said trench; removing said oxide liningexcept for a remaining portion of said oxide lining that is adjacentsaid remaining portion of said first polysilicon layer; anisotropicallyetching a top surface of said remaining portion of said firstpolysilicon layer so as to depress said top surface to a level below atop surface of said remaining portion of said oxide lining; heating saiddevice so as to form a first oxide layer on said top surface of saidfirst polysilicon layer and a second oxide layer on exposed portions ofsaid sidewalls of said trench; removing said first oxide layer whileleaving a portion of said second oxide layer on the sidewalls of saidtrench; and depositing a second polysilicon layer in said trench.
 2. Theprocess of claim 1 wherein said second polysilicon layer extends oversaid surface of said semiconductor material, said process comprisingetching said second polysilicon layer such that a surface of said secondpolysilicon layer is substantially coplanar with said surface of saidsemiconductor material.
 3. A process of fabricating a trenchsemiconductor device comprising: providing a semiconductor material;forming a mask layer on a surface of said semiconductor material;forming a opening in said mask layer; forming a trench in saidsemiconductor material by etching said semiconductor material throughsaid opening; forming an oxide lining on a bottom and sidewalls of saidtrench; depositing a photoresist layer in said trench; washing saidphotoresist layer such that a remaining portion of said photoresistlayer remains over said oxide lining at the bottom of said trench;removing said oxide lining except for a remaining portion of said oxidelining that is adjacent said remaining portion of said photoresistlayer, thereby leaving exposed portions of said sidewalls of saidtrench; removing said remaining portion of said photoresist layer;growing a gate oxide layer on said exposed portions of said sidewalls ofsaid trench; and depositing a polysilicon layer in said trench.